An oxygen absorber which utilizes an oxidation reaction of a reduced metal, such as metallic iron, has been widely known. Other than the reduced metal, this oxygen absorber comprises a metal halide as an accelerator for promoting the oxidation reaction of the reduced metal.
In order to mainly preserve the freshness of food, this type of oxygen absorber is placed together with a preserved substance, such as food, in a container or a packing material, or the oxygen absorber is contained within a container or a sheet material, which is then used to package a substance to be preserved.
The Japanese Patent (Kokoku) Publication No. SHO 56-33980 discloses an oxygen absorber of metal powder coated with a metal halide. Since a deoxidation reaction of a reduced metal requires the existence of water, this type of oxygen absorber is used to preserve a food containing a large amount of water (high water activity, a.sub.w) which is capable of utilizing water transpired from the preserved substance. An oxygen absorber of that uses water vaporized from the preserved substance is known as a moisture-dependent oxygen absorber.
On the other hand, the Japanese Patent Kokoku Publication No. SHO 57-31449 discloses an oxygen absorber for preserving dried food having low water content (low water activity) wherein a water provider, such as an inorganic filler, impregnated with water is included in the oxygen absorbing composition and moisture necessary for an oxygen absorbing reaction is supplied therefrom. In order to preserve a substance having a low water content (i.e., having a low water activity (a.sub.w)) such as dried food, a water-supplying component is included in an oxygen absorber in order to supplement water necessary for the oxygen-absorbing reaction. Since this type of oxygen absorber holds moisture in itself, it is known as a self-reacting oxygen absorber. A self-reacting type oxygen absorber is capable of absorbing oxygen without depending on water from the preserved substance.
The recent practice has been to package a preserved substance by using a sheet-shaped packing material which is made by kneading an oxygen absorber with a resin and forming the kneaded mixture into a sheet or film. However, if the self-reacting type of oxygen absorber which holds water is used to preserve a substance having a low water content, the transfer of water from the water-holding component to the preserved substance cannot be prevented. This results in problems in that the water spoils the taste of the preserved substance (due to humidity), changes the properties of the preserved substance (i.e., solidification of powder), causes a chemical change (or hydrolysis), or causes propagation of bacteria. In some cases, another problem is caused in that the oxygen absorber loses the water necessary for deoxidation and then becomes incapable of absorbing oxygen.
Furthermore, as the self-reacting type of oxygen absorber which holds water is heated when it is kneaded with a resin in order to be formed into a sheet, there is a problem in that not only water in the water-holding component evaporates, but also the evaporated water generates bubbles within the resin sheet, the surface of the resin sheet is made uneven and good surface properties of the sheet cannot be obtained.
On the other hand, when an oxygen absorber holding no water (water-dependent type) is kneaded with a resin to obtain a sheet material, it is necessary to perform the treatment for making the oxygen absorber hold water. However, if the resin sheet is made to hold water, this oxygen-absorbing resin sheet takes over the above-described defects of the self-reacting oxygen absorber.
Accordingly, the conventional oxygen-absorbing sheet is limited for the practical use to the type which itself contains no water and which is capable of absorbing oxygen by acquiring from a food having a high water content which contains moisture necessary for the oxygen-absorbing reaction. This oxygen-absorbing sheet is usually applied to a food containing a large amount of water.
Because of the reasons described above, those skilled in the art have had difficulties in realizing an oxygen-absorbing sheet capable of causing the deoxidation in a low-water environment. In other words, it has been difficult to preserve items having a low water content, such as a dried food, in good condition in a low oxygen environment by using the conventional oxygen absorber.
Accordingly, an object of the present invention is to provide an oxygen absorber and an oxygen-absorbing composition capable of sufficiently exhibiting the oxygen-absorbing function even in a low humidity environment with relative humidity of no more than 70% without any water being supplied, and which is capable of maintaining product quality when applied to items requiring a low-humidity environment for preservation. Another object of this invention is to provide an oxygen-absorbing resin composition having good thermoformability made by embedding an oxygen absorber in a thermoplastic resin. Furthermore, another object of the invention is to provide an oxygen absorber packet formed by wrapping such oxygen-absorbing composition and a preservation method utilizing such oxygen absorber packet.
A further object of this invention is to provide an oxygen-absorbing member and packing material in the form of a sheet or film, and a packing container, which are capable of exhibiting a sufficient oxygen-absorbing function in the low humidity atmosphere. Another object of this invention is to provide a package to preserve a substance having a low water content, which requires the low-humidity preservation condition, in the atmosphere of low oxygen concentration. A further object of this invention is to provide a method for preserving a substance having a low water content, which makes it possible to maintain the substance having a low water content in the atmosphere of low oxygen concentration by using the above-mentioned oxygen-absorbing composition.